For a conventional magnetic pole estimating method, there has widely been used a method of calculating an induced voltage which is proportional to the speed of an electric motor based on an input voltage and a current of the electric motor and estimating a speed as reported in IEEE Paper D., Vol. 108, No. 12, 1988 “Method of Controlling Adaptive Current of Brushless DC motor having Parameter Identifying Function”. Moreover, there has been proposed 1996 IEEE Industry Applications Society National Convention No. 170 “Zero Speed Torque Control of Salient-Pole Type Synchronous Motor using Sensorless System”. According to this system, an AC signal is superposed on a voltage command value and a detecting current is subjected to an FFT analysis to detect a rotating speed of an electric motor and a position of a magnetic pole.